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17 16 RFC 6997
Internet Engineering Task Force M. Goyal, Ed.
Internet-Draft University of Wisconsin
Intended status: Experimental Milwaukee
Expires: August 7, 2013 E. Baccelli
M. Philipp
INRIA
A. Brandt
Sigma Designs
J. Martocci
Johnson Controls
February 3, 2013
Reactive Discovery of Point-to-Point Routes in Low Power and Lossy
Networks
draft-ietf-roll-p2p-rpl-16
Abstract
This document specifies a point-to-point route discovery mechanism,
complementary to the RPL core functionality. This mechanism allows
an IPv6 router to discover "on demand" routes to one or more IPv6
routers in the LLN such that the discovered routes meet specified
metrics constraints.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 7, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Functional Overview . . . . . . . . . . . . . . . . . . . . . 6
6. P2P Route Discovery Mode Of Operation . . . . . . . . . . . . 9
6.1. Setting a P2P Mode DIO . . . . . . . . . . . . . . . . . . 9
7. New RPL Control Message Options . . . . . . . . . . . . . . . 12
7.1. P2P Route Discovery Option (P2P-RDO) . . . . . . . . . . . 13
7.2. Data Option . . . . . . . . . . . . . . . . . . . . . . . 16
8. The Discovery Reply Object (DRO) . . . . . . . . . . . . . . . 17
8.1. Secure DRO . . . . . . . . . . . . . . . . . . . . . . . . 19
8.2. Setting a P2P-RDO Carried in a Discovery Reply Object . . 19
9. P2P-RPL Route Discovery By Creating a Temporary DAG . . . . . 20
9.1. Joining a Temporary DAG . . . . . . . . . . . . . . . . . 20
9.2. Trickle Operation For P2P Mode DIOs . . . . . . . . . . . 20
9.3. Processing a P2P Mode DIO . . . . . . . . . . . . . . . . 23
9.4. Additional Processing of a P2P Mode DIO At An
Intermediate Router . . . . . . . . . . . . . . . . . . . 24
9.5. Additional Processing of a P2P Mode DIO At The Target . . 25
9.6. Processing a DRO At An Intermediate Router . . . . . . . . 26
9.7. Processing a DRO At The Origin . . . . . . . . . . . . . . 28
10. The Discovery Reply Object Acknowledgement (DRO-ACK) . . . . . 29
11. Packet Forwarding Along a Route Discovered Using P2P-RPL . . . 30
12. Interoperability with Core RPL . . . . . . . . . . . . . . . . 30
13. Security Considerations . . . . . . . . . . . . . . . . . . . 31
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
14.1. Additions to Mode of Operation . . . . . . . . . . . . . . 32
14.2. Additions to RPL Control Message Options . . . . . . . . . 32
14.3. Additions to RPL Control Codes . . . . . . . . . . . . . . 33
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
16.1. Normative References . . . . . . . . . . . . . . . . . . . 34
16.2. Informative References . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
Targeting Low power and Lossy Networks (LLNs), the IPv6 Routing
Protocol for LLNs (RPL) [RFC6550] provides paths along a Directed
Acyclic Graph (DAG) rooted at a single router in the network.
Establishment and maintenance of a DAG is performed by routers in the
LLN using Destination-Oriented DAG (DODAG) Information Object (DIO)
messages. When two arbitrary routers (neither of which is the DAG's
root) need to communicate, the data packets are restricted to travel
only along the links in the DAG. Such point-to-point (P2P) routing
functionality may not be sufficient for several Home and Building
Automation applications [RFC5826] [RFC5867] due to the following
reasons:
o The need to pre-establish routes: each potential destination in
the network must declare itself as such ahead of the time a source
needs to reach it.
o The need to route only along the links in the DAG: A DAG is built
to optimize the routing cost to reach the root. Restricting P2P
routes to use only the in-DAG links may result in significantly
suboptimal routes and severe traffic congestion near the DAG root.
This document describes an extension to core RPL (i.e., the RPL
functionality described in [RFC6550]) that enables an IPv6 router in
the LLN to discover routes to one or more IPv6 routers in the LLN "on
demand". The discovered routes may not be the best available but are
guaranteed to meet the specified routing metric constraints. Thus,
such routes are considered "good enough" from the application's
perspective. This reactive P2P route discovery mechanism is
henceforth referred to as P2P-RPL.
A mechanism to measure the end-to-end cost of an existing route is
specified in [I-D.ietf-roll-p2p-measurement]. As discussed in
Section 4, measuring the end-to-end cost of an existing route may
help decide whether to initiate the discovery of a better route using
P2P-RPL and the metric constraints to be used for this purpose.
This document is presented as an Experimental specification to
facilitate P2P-RPL's deployment in LLN scenarios where reactive P2P
route discovery is considered useful or necessary. It is anticipated
that, once sufficient operational experience has been gained, this
specification will be revised to progress it on to the Standards
Track. Experience reports regarding P2P-RPL implementation and
deployment are encouraged particularly with respect to:
o The values in the default DODAG Configuration Option
(Section 6.1);
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o The rules governing Trickle operation (Section 9.2);
o The utility and the implementation complexity of the Data Option
(Section 7.2) that provides a facility to piggyback time-critical
application data on the routing messages;
o The utility and the implementation complexity of allowing multiple
Target addresses in a P2P-RPL route discovery.
2. The Use Cases
One use case, common in home [RFC5826] and commercial building
[RFC5867] environments, involves a device (say a remote control or an
airduct controller) that suddenly needs to communicate with another
device (say a lamp or a humidity sensor) to which it does not already
have a route. In this case, the remote control (or the airduct
controller) must be able to discover a route to the lamp (or the
humidity sensor) "on demand".
Another use case, common in a commercial building environment,
involves a large LLN deployment where P2P communication along a
particular DAG among hundreds (or thousands) of routers creates
severe traffic congestion near that DAG's root, and thus routes
across this DAG are desirable.
Other use cases involve scenarios where energy or latency constraints
are not satisfied by the P2P routes along an existing DAG because
they involve traversing many more intermediate routers than necessary
to reach the destination.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
Additionally, this document uses terminology from [RFC6550]. This
document introduces the following terms:
Origin : The IPv6 router initiating the P2P-RPL route discovery.
Target : The IPv6 router at the other end point of the P2P route(s)
to be discovered. A P2P-RPL route discovery can discover routes to
multiple Targets at the same time.
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Intermediate Router: An IPv6 router that is neither the Origin nor a
Target.
Forward direction: The direction from the Origin to the Target.
Reverse direction: The direction from the Target to the Origin.
Forward Route: A route in the Forward direction.
Reverse Route: A route in the Reverse direction.
Bidirectional Route: A route that can be used in both Forward and
Reverse directions.
Source Route: A complete and ordered list of routers that can be used
by a packet to travel from a source to a destination node.
Hop-by-hop Route: The route characterized by each router on the route
using its routing table to determine the next hop on the route.
4. Applicability
A route discovery using P2P-RPL may be performed by an Origin when no
route exists between itself and the Target(s) or when the existing
routes do not satisfy the application requirements. P2P-RPL is
designed to discover Hop-by-hop or Source Routes to one or more
Targets such that the discovered routes meet the specified
constraints. In some application contexts, the constraints that the
discovered routes must satisfy are intrinsically known or can be
specified by the application. For example, an Origin that expects
its Targets to be less than 5 hops away may use "hop-count < 5" as
the constraint. In other application contexts, the Origin may need
to measure the cost of the existing route to a Target to determine
the constraints. For example, an Origin that measures the total ETX
along its current route to a Target to be 20 may use "ETX < x*20",
where x is a fraction that the Origin decides, as the constraint. A
mechanism to measure the cost of an existing route between two IPv6
routers is specified in [I-D.ietf-roll-p2p-measurement]. If there is
no existing route between the Origin and the Target(s) or the cost
measurement for the existing routes fails, the Origin will have to
guess the constraints to be used in the initial route discovery.
Once, the initial route discovery succeeds or fails, the Origin will
have a better estimate for the constraints to be used in the
subsequent route discovery.
P2P-RPL may result in discovery of better P2P routes than the ones
available along a global DAG designed to optimize routing cost to the
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DAG's root. The improvement in route quality depends on a number of
factors including the network topology, the "distance" between the
Origin and the Target (in terms of the routing metrics in use) and
the prevalent conditions in the network. In general, a P2P-RPL route
may be better than the one along a global DAG if the Origin and the
Target are nearby. Similarly, a P2P-RPL route may not be much better
than the one along a global DAG if the Origin and the Target are far
apart. Note that, even when P2P-RPL routes are not much better than
those along a global DAG, P2P-RPL routes may still be able to avoid
congestion that might occur near the root if the routing takes place
only along a global DAG. In general, the costs associated with a
P2P-RPL route discovery (in terms of the control messages, mostly
DIOs, generated) increases with the distance between the Origin and
the Target. However, it is possible to limit the cost of route
discovery by carefully setting the routing constraints, the Trickle
parameters (that govern the DIO generation) and the lifetime of the
temporary DAG created for the route discovery. A network designer
may take into consideration both the benefits (potentially better
routes; no need to maintain routes proactively; avoid congestion near
the global DAG's root) and costs when using P2P-RPL. The latency
associated with a P2P-RPL route discovery again depends on the
distance between the Origin and the Target and the Trickle
parameters.
Like core RPL [RFC6550], P2P-RPL operation requires links to have
bidirectional reachability. Routers participating in a P2P-RPL route
discovery must ensure that
o Links that do not have bidirectional reachability do not become
part of the route being discovered; and
o IPv6 addresses belonging to egress-only or ingress-only interfaces
do not become part of the route being discovered.
5. Functional Overview
This section contains a high level description of P2P-RPL.
A P2P-RPL route discovery takes place by forming a DAG rooted at the
Origin. As is the case with core RPL, P2P-RPL uses IPv6 link-local
multicast DIO messages to establish a DAG. However, unlike core RPL,
this DAG is temporary in nature and routers in the DAG leave once the
DAG's life time is over. The sole purpose of DAG creation is to
discover routes to the Target(s) and DIOs serve as the route
discovery messages. Each router joining the DAG determines a rank
for itself in the DAG and ignores the subsequent DIOs received from
lower (higher in numerical value) ranked neighbors. Thus, the route
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discovery messages propagate away from the Origin rather than return
back to it. As in core RPL, DIO generation at a router is controlled
by a Trickle timer [RFC6206] that allows a router to avoid generating
unnecessary messages while providing protection against packet loss.
P2P-RPL also uses the routing metrics [RFC6551], objective functions
and packet forwarding framework [RFC6554][RFC6553] developed for core
RPL.
An Origin may use P2P-RPL to discover routes to one or more Target(s)
identified by one or more unicast/multicast addresses. P2P-RPL
allows for the discovery of one Hop-by-hop Route or up to four Source
Routes per Target. The discovered routes are guaranteed to meet the
specified routing metric constraints but may not be the best
available. P2P-RPL may fail to discover any route if the specified
routing constraints are overly strict. P2P-RPL allows an Origin to
piggyback time-critical application data on the DIO messages for
delivery to the Target(s).
The Origin initiates a P2P-RPL route discovery by forming a temporary
DAG rooted at itself. The DIOs used to create the temporary DAG are
identified by a new Mode of Operation (P2P Route Discovery mode
defined in Section 6). The DIOs listing the P2P Route Discovery mode
as the Mode of Operation are henceforth referred to as the P2P mode
DIOs. A P2P mode DIO always carries exactly one P2P Route Discovery
Option (defined in Section 7.1) in which the Origin specifies the
following information:
o The IPv6 address of a Target. This could be a unicast address or
a multicast address. Any additional Targets may be specified by
including one or more RPL Target Options [RFC6550] inside the DIO.
o The nature of the route(s) to be discovered: Hop-by-hop or Source
Routes. This specification allows for the discovery of one Hop-
by-hop Route or up to four Source Routes per Target.
o The desired number of routes (if Source Routes are being
discovered).
o Whether the Target(s) should send Discovery Reply Object (DRO)
messages (defined in Section 8) back to the Origin on receiving a
DIO message. A DRO message carries a discovered Source Route back
to the Origin or establishes a Hop-by-hop Route between the Origin
and the Target. By not allowing the generation of DRO messages,
an Origin can use P2P-RPL as purely a mechanism to deliver time-
critical application data to the Target(s).
A P2P Route Discovery Option also accumulates a route from the Origin
to a Target as the routers join the temporary DAG.
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A P2P mode DIO MAY also carry:
o One or more Metric Container Options to specify:
* The relevant routing metrics.
* The constraints that the discovered route must satisfy. These
constraints also limit how far the DIOs message may travel.
o One or more RPL Target options to specify additional unicast or
multicast Targets.
o One Data Option (defined in Section 7.2) to carry time-critical
application-level data to be delivered to the Target(s).
As the routers join the temporary DAG, they keep track of the best
route(s) (so far from the Origin) they have seen and advertise these
routes, along with the corresponding routing metrics, in their P2P
mode DIOs. A router, including the Target(s), discards a received
P2P mode DIO if the aggregated routing metrics on the route
advertised by the DIO do not satisfy the listed constraints. These
constraints can be used to limit the propagation of P2P mode DIO
messages. A router may also discard a received P2P mode DIO if it
does not wish to be a part of the discovered route due to limited
resources or due to policy reasons.
When a Target receives a P2P mode DIO, it forwards the data in the
Data Option, if present, to the higher layer. Since the links in the
discovered route have bidirectional reachability (Section 7.1), the
Target may remember the discovered route for use as a Source Route to
reach the Origin. If the Origin has requested DRO messages to be
sent back, the Target may select the route contained in the received
DIO for further processing as described next. This document does not
specify a particular method for the Target to use to select a route
for further processing. Example methods include selecting any route
that meets the constraints or selecting the best route(s) discovered
over a certain time period.
If one or more Source Route(s) are being discovered, the Target sends
the selected Source Route(s) to the Origin via DRO messages with one
DRO message carrying one discovered route. On receiving a DRO
message, the Origin stores the discovered route in its memory. This
specification allows the Origin to discover up to four Source Routes
per Target, thereby allowing the Origin to have sufficient ready-to-
use alternatives should one or more of these routes fail. If a Hop-
by-hop Route is being discovered, the Target sends a DRO message
containing the selected route to the Origin. The DRO message travels
back to the Origin along the selected route, establishing state for
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the Forward Route in the routers on the path. The Target may include
a Data Option in a DRO message to deliver any time-critical
application data to the Origin.
The Target may request the Origin to acknowledge the receipt of a DRO
message by sending back a DRO Acknowledgement (DRO-ACK) message
(defined in Section 10). The Origin unicasts a DRO-ACK message to
the Target. If the Target does not receive the requested DRO-ACK
within a certain time interval of sending a DRO, it resends the DRO
message (up to a certain number of times) carrying the same route as
before.
The use of trickle timers to delay the propagation of DIO messages
may cause some nodes to generate these messages even when the desired
routes have already been discovered. In order to preempt the
generation of such unnecessary messages, the Target may set a "Stop"
flag in the DRO message to let the nodes in the LLN know about the
completion of the route discovery process. The routers receiving
such a DRO should not generate any more DIOs for this temporary DAG.
Neither should they process any received DIOs for this temporary DAG
in future. However, such routers must still process the DROs
received for this temporary DAG.
6. P2P Route Discovery Mode Of Operation
This section specifies a new RPL Mode of Operation (MOP), P2P Route
Discovery Mode (or P2P mode, for short), with value TBD1. A DIO
message, listing P2P mode as the MOP, is identified as performing a
P2P-RPL route discovery by creating a temporary DAG. A P2P mode DIO
MUST carry exactly one P2P Route Discovery Option (specified in
Section 7.1).
6.1. Setting a P2P Mode DIO
The Base Object in a P2P mode DIO message MUST be set in the
following manner:
o RPLInstanceID: RPLInstanceID MUST be a local value as described in
Section 5.1 of [RFC6550]. The Origin MUST NOT reuse an
RPLInstanceID for a route discovery if its previous route
discovery using this RPLInstanceID might still be going on. As
described in Section 7.1, the Life Time parameter in the P2P Route
Discovery Option specifies the time duration the route discovery
lasts. So, the Origin MUST NOT reuse an RPLInstanceID in a route
discovery until the Life Time of its previous route discovery
using this RPLInstanceID is over. When initiating a new route
discovery to a particular Target, the Origin MUST NOT reuse the
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RPLInstanceID used in a previous route discovery to this Target if
the previously discovered routes might still exist. The Default
Lifetime and Lifetime Unit parameters in the DODAG Configuration
Option specify the lifetime of the state the routers, including
the Origin and the Target, maintain for a Hop-by-hop or a Source
Route discovered using P2P-RPL. Thus, an Origin can safely reuse
an RPLInstanceID to discover a new route to a Target if the
lifetime of all previously discovered routes to this Target using
this RPLInstanceID is over.
o Version Number: MUST be set to zero. The temporary DAG used for
P2P-RPL route discovery does not exist long enough to have new
versions (the Life Time parameter inside the P2P Route Discovery
Option, defined in Section 7.1, specifies the life time of the
temporary DAG).
o Grounded (G) Flag: This flag MUST be set to one. Unlike a global
RPL instance, the concept of a floating DAG, used to provide
connectivity within a sub-DAG detached from a grounded DAG, does
not apply to a local RPL instance. Hence, an Origin MUST always
set the G flag to one when initiating a P2P-RPL route discovery.
Further, clause 3 of Section 8.2.2.2 in [RFC6550] does not apply
and a node MUST NOT initiate a new DAG if it does not have any
parent left in a P2P-RPL DAG.
o Mode of Operation (MOP): MUST be set to TBD1, corresponding to P2P
Route Discovery mode.
o DTSN: MUST be set to zero on transmission and ignored on
reception.
o DODAGPreference (Prf): This field MUST be set to zero (least
preferred).
o DODAGID: This field MUST be set to an IPv6 address of the Origin.
o The other fields in the DIO Base Object can be set in the desired
fashion as per the rules described in [RFC6550].
A received P2P mode DIO MUST be discarded if it does not follow the
above-listed rules regarding the RPLInstanceID, Version Number, G
flag, MOP and Prf fields inside the base object.
The DODAG Configuration Option, inside a P2P mode DIO MUST be set in
the following manner:
o The Origin MUST set the MaxRankIncrease parameter to zero to
disable local repair of the temporary DAG. A received P2P mode
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DIO MUST be discarded if the MaxRankIncrease parameter inside the
DODAG Configuration Option is not zero.
o The Origin SHOULD set the Trickle parameters
(DIOIntervalDoublings, DIOIntervalMin, DIORedundancyConstant) as
recommended in Section 9.2.
o The Origin sets the Default Lifetime and Lifetime Unit parameters
to indicate the lifetime of the state the routers, including the
Origin and the Target(s), maintain for a Hop-by-hop or a Source
Route discovered using P2P-RPL.
o The Origin sets the other fields in the DODAG Configuration
Option, including the OCP identifying the Objective function, in
the desired fashion as per the rules described in [RFC6550].
o An Intermediate Router (or a Target) MUST set various fields in
the DODAG Configuration Option in the outgoing P2P mode DIOs to
the values they had in the incoming P2P mode DIOs for this DAG.
A default DODAG Configuration Option comes in effect if a P2P mode
DIO does not carry an explicit one. The default DODAG Configuration
Option has the following parameter values:
o Authentication Enabled: 0
o DIOIntervalMin: 6, which translates to 64ms as the value for Imin
parameter in Trickle operation. This value is roughly one order
of magnitude larger than the typical transmission delay on IEEE
802.15.4 links and corresponds to the recommendation in
Section 9.2 for well-connected topologies.
o DIORedundancyConstant: 1. See the discussion in Section 9.2.
o MaxRankIncrease: 0 (to disable local repair of the temporary DAG).
o Default Lifetime: 0xFF, to correspond to infinity.
o Lifetime Unit: 0xFFFF, to correspond to infinity.
o Objective Code Point: 0, i.e., OF0 [RFC6552] is the default
objective function.
o The remaining parameters have default values as specified in
[RFC6550].
Individual P2P-RPL deployments are encouraged to share their
experience with these default values with ROLL working group to help
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guide the development of standards track version of the protocol.
The routing metrics and constraints [RFC6551] used in P2P-RPL route
discovery are included in one or more Metric Container Options
[RFC6550] inside the P2P mode DIO. Note that a DIO need not include
a Metric Container if OF0 is the objective function in effect. In
that case, a P2P mode DIO may still specify an upper limit on the
maximum rank, that a router may have in the temporary DAG, inside the
P2P Route Discovery Option (described in Section 7.1).
A P2P mode DIO:
o MUST carry one (and only one) P2P Route Discovery Option
(described in Section 7.1). The P2P Route Discovery Option allows
for the specification of one unicast or multicast address for the
Target. A received P2P mode DIO MUST be discarded if it does not
contain exactly one P2P Route Discovery Option.
o MAY carry one or more RPL Target Options to specify additional
unicast/multicast addresses for the Target.
o MAY carry one or more Metric Container Options to specify routing
metrics and constraints.
o MAY carry one Data Option (described in Section 7.2) containing
time-critical application data to be delivered to the Target(s).
A received P2P mode DIO MUST be discarded if it contains multiple
Data Options.
o MAY carry one or more Route Information Options [RFC6550]. In the
context of P2P-RPL, a Route Information Option advertizes to the
Target(s) the Origin's connectivity to the prefix specified in the
option.
o MAY carry one or more Prefix Information Options subject to the
usage and rules specified in Section 6.7.10 in [RFC6550].
7. New RPL Control Message Options
This document defines two new RPL control message options: the P2P
Route Discovery Option and the Data Option.
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7.1. P2P Route Discovery Option (P2P-RDO)
-
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD2 | Option Length |R|H| N | Compr | L |MaxRank/NH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| TargetAddr |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address[1..n] |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of P2P Route Discovery Option (P2P-RDO)
The format of a P2P Route Discovery Option (P2P-RDO) is illustrated
in Figure 1. A P2P mode DIO and a DRO (defined in Section 8) message
MUST carry exactly one P2P-RDO. A P2P-RDO consists of the following
fields:
o Option Type: TBD2.
o Option Length: 8-bit unsigned integer, representing the length in
octets of the option, not including the Option Type and Option
Length fields.
o Reply (R): The Origin sets this flag to one to allow the Target(s)
to send DRO messages back to the Origin. If this flag is zero, a
Target MUST NOT generate any DRO message.
o Hop-by-hop (H): This flag is valid only if the R flag is set to
one. The Origin sets this flag to one if it desires Hop-by-hop
Routes. The Origin sets this flag to zero if it desires Source
Routes. This specification allows for the establishment of one
Hop-by-hop route or up to four Source Routes per Target. The Hop-
by-hop Route is established in the Forward direction, i.e. from
the Origin to the Target. This specification does not allow for
the establishment of Hop-by-hop Routes in the Reverse direction.
o Number of Routes (N): This field is valid only if the R flag is
one and H flag is zero, i.e. the Targets are allowed to generate
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DRO messages carrying discovered Source Routes back to the Origin.
In this case, the value in the N field plus one indicates the
number of Source Routes that each Target should convey to the
Origin. When Hop-by-hop Routes are being discovered, the N field
MUST be set to zero on transmission and ignored on reception.
o Compr: 4-bit unsigned integer indicating the number of prefix
octets that are elided from the Target field and the Address
vector. For example, Compr value will be zero if full IPv6
addresses are carried in the Target field and the Address vector.
o Life Time (L): A 2-bit field that indicates the life time of the
temporary DAG, i.e., the exact duration a router joining the
temporary DAG MUST maintain its membership in the DAG. The
mapping between the values in this field and the life time of the
temporary DAG is as follows:
* 0x00: 1 second;
* 0x01: 4 seconds;
* 0x02: 16 seconds;
* 0x03: 64 seconds;
The Origin sets this field based on its expectation regarding the
time required for the route discovery to complete, which includes
the time required for the DIOs to reach the Target(s) and the DROs
to travel back to the Origin. The time required for the DIOs to
reach the Target(s) would in turn depend on the Trickle parameters
(Imin and the redundancy constant) as well as the expected
distance (in terms of hops and/or ETX) to the Target(s). While
deciding the temporary DAG's lifetime, the Origin should also take
in account the fact that all routers joining the temporary DAG
would need to stay in the DAG for this much time.
o MaxRank/NH:
* When a P2P-RDO is included in a P2P mode DIO, this field
indicates the upper limit on the integer portion of the rank
(calculated using the DAGRank() macro defined in [RFC6550])
that a router may have in the temporary DAG being created. An
Intermediate Router MUST NOT join a temporary DAG being created
by a P2P mode DIO if the integer portion of its rank would be
equal to or higher (in numerical value) than the MaxRank limit.
A Target can join the temporary DAG at a rank whose integer
portion is equal to the MaxRank. A router MUST discard a
received P2P mode DIO if the integer part of the advertized
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rank equals or exceeds the MaxRank limit. A value 0 in this
field indicates that the MaxRank is infinity.
* When a P2P-RDO is included in a DRO message, this field
indicates the index of the next hop address inside the Address
vector.
o TargetAddr: An IPv6 address of the Target after eliding Compr
number of prefix octets. When the P2P-RDO is included in a P2P
mode DIO, this field may contain a unicast address or a multicast
address. Any additional Target addresses can be specified by
including one or more RPL Target Options [RFC6550] in the DIO.
When the P2P-RDO is included in a DRO, this field MUST contain a
unicast IPv6 address of the Target generating the DRO.
o Address[1..n]: A vector of IPv6 addresses representing a complete
route so far in the Forward direction:
* Each element in the Address vector has size (16 - Compr) octets
and MUST contain a valid IPv6 address with first Compr octets
elided.
* The total number of elements inside the Address vector is given
by n = (Option Length - 2 - (16 - Compr))/(16 - Compr).
* IPv6 addresses of ingress-only (or egress-only) router
interfaces MUST NOT be added to the Address vector. This
allows the route accumulated in the Address vector to be a
Bidirectional Route that can be used by a Target to send a DRO
message to the Origin.
* The Address vector MUST carry the accumulated route in the
Forward direction, i.e., the first element in the Address
vector must contain the IPv6 address of the router next to the
Origin and so on.
* The Origin and Target addresses MUST NOT be included in the
Address vector.
* A router adding its address to the vector MUST ensure that any
of its addresses do not already exist in the vector. A Target
specifying a complete route in the Address vector MUST ensure
that the vector does not contain any address more than once.
* The Address vector MUST NOT contain any multicast addresses.
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7.2. Data Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD3 | Option Length | UpperLayerPrt | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Figure 2: Format of Data Option
The format of a Data Option is illustrated in Figure 2. A P2P mode
DIO and a DRO (defined in Section 8) message MAY carry one Data
Option. A P2P-RDO consists of the following fields:
o Option Type: TBD3.
o Option Length: An 8-bit unsigned integer, representing the length
in octets of the option, not including the Option Type and Option
Length fields.
o Upper Layer Protocol: An 8-bit field that identifies the upper
layer protocol header with which the information in the Data field
starts. The protocol identifiers used in this field are same as
those defined in IANA's "Protocol Numbers" registry [PROTOCOL].
o Data: If the Data Option is contained in a DIO, this field
contains application data to be delivered to the Target(s). If
the Data Option is contained in a DRO, this field contains
application data to be delivered to the Origin.
If the Origin chooses to include a Data Option inside its DIO, it
MUST include the same Data Option in all its future DIO transmissions
for this temporary DAG. An Intermediate Router MUST NOT modify the
Data Option received inside a parent's DIO and MUST include this Data
Option in all its future DIO transmissions for this temporary DAG.
The same is true for a Target that needs to propagate the DIOs
further (required when the route discovery involves multiple
Targets). If a Target chooses to include a Data Option inside a DRO,
it MUST include the same Data Option in all retransmissions of this
DRO message and MUST NOT include a different Data Option in any other
DRO messages it generates for this route discovery. Also, an
Intermediate Router, which needs to forward a received DRO message
further, MUST include in the forwarded message a verbatim copy of the
Data Option found inside the received message.
Note that the data inside a Data Option has the same level of
security as the DIO/DRO message it is part of. A P2P-RPL deployment
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SHOULD take in consideration the security requirements of the data
being sent inside the Data Options when deciding the overall security
requirements. Further, note that P2P-RPL does not guarantee
successful delivery of the data contained in a Data Option.
8. The Discovery Reply Object (DRO)
This section defines two new RPL Control Message types, the Discovery
Reply Object (DRO), with code TBD4, and the Secure DRO, with code
TBD5. A DRO serves one of the following functions:
o Carry a discovered Source Route from a Target to the Origin;
o Establish a Hop-by-hop Route as it travels from a Target to the
Origin.
A DRO message MAY serve the function of letting the routers in the
LLN know that a P2P-RPL route discovery is complete and no more DIO
messages need to be generated for the corresponding temporary DAG. A
DRO message MAY also carry time-critical application data from the
Target to the Origin in a Data Option. A DRO message MUST carry one
(and only one) P2P-RDO whose TargetAddr field MUST contain a unicast
IPv6 address of the Target that generates the DRO. A DRO message
travels from the Target to the Origin via link-local multicast along
the route specified inside the Address vector in the P2P-RDO, as
included in the DRO.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID | Version |S|A|Seq| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DODAGID |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Figure 3: Format of the base Discovery Reply Object (DRO)
The format of the base Discovery Reply Object (DRO) is shown in
Figure 3. A base DRO consists of the following fields:
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o RPLInstanceID: The RPLInstanceID of the temporary DAG used for
route discovery.
o Version: The Version of the temporary DAG used for route
discovery. Since a temporary DAG always has value zero for the
Version, this field MUST always be set to zero.
o Stop (S): This flag, when set to one by a Target, indicates that
the P2P-RPL route discovery is over. All the routers receiving
such a DRO, including the ones not listed in the route carried
inside P2P-RDO,
* SHOULD NOT process any more DIOs received for this temporary
DAG;
* SHOULD NOT generate any more DIOs for this temporary DAG;
* SHOULD cancel any pending DIO transmission for this temporary
DAG.
Note that the Stop flag serves to stop further DIO generation/
processing for a P2P-RPL route discovery but it does not affect
the processing of DRO messages at either the Origin or the
Intermediate Routers. In other words, a router (the Origin or an
Intermediate Router) MUST continue to process the DRO messages
even if an earlier DRO message (with the same RPLInstanceID and
DODAGID fields) had the Stop flag set to one. When set to zero,
this flag does not imply any thing and MUST be ignored on
reception.
o Ack Required (A): This flag, when set to one by the Target,
indicates that the Origin MUST unicast a DRO-ACK message (defined
in Section 10) to the Target when it receives the DRO.
o Sequence Number (Seq): This 2-bit field indicates the sequence
number for the DRO. This field is relevant when the A flag is set
to one, i.e., the Target requests an acknowledgement from the
Origin for a received DRO. The Origin includes the RPLInstanceID,
the DODAGID and the Sequence Number of the received DRO inside the
DRO-ACK message it sends back to the Target.
o Reserved: These bits are reserved for future use. These bits MUST
be set to zero on transmission and MUST be ignored on reception.
o DODAGID: The DODAGID of the temporary DAG used for route
discovery. The DODAGID also identifies the Origin. The
RPLInstanceID, the Version and the DODAGID together uniquely
identify the temporary DAG used for route discovery and can be
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copied from the DIO message advertizing the temporary DAG.
o Options: The DRO message:
* MUST carry one (and only one) P2P-RDO that MUST specify a
complete route between the Target and the Origin;
* MAY carry one or more Metric Container Options that contains
the aggregated routing metrics values for the route specified
in P2P-RDO;
* MAY carry one Data Option to carry any time-critical
application data to the Origin, subject to the following
conditions: if a Target chooses to include a Data Option inside
a DRO,
+ it MUST include the same Data Option in all retransmissions
of this DRO message and
+ it MUST NOT include a different Data Option in any other DRO
messages it generates for this route discovery.
The Target MAY repeat the same Data Option in multiple DRO
messages it generates for a particular route discovery.
A received DRO message MUST be discarded if it does not contain
exactly one P2P-RDO or if it contains multiple Data Options.
8.1. Secure DRO
A Secure DRO message follows the format in Figure 7 of [RFC6550],
where the base format is the base DRO shown in Figure 3.
8.2. Setting a P2P-RDO Carried in a Discovery Reply Object
A Discovery Reply Object MUST carry one (and only one) P2P-RDO, which
MUST be set as defined in Section 7.1. Specifically, the following
fields MUST be set as specified next:
o Reply (R): This flag MUST be set to zero on transmission and
ignored on reception.
o Hop-by-Hop (H): The H flag in the P2P-RDO included in a DRO
message MUST have the same value as the H flag in the P2P-RDO
inside the corresponding DIO message.
o Number of Routes (N): This field MUST be set to zero on
transmission and ignored on reception.
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o Life Time (L): This field MUST be set to zero on transmission and
ignored on reception.
o MaxRank/NH: This field indicates the index of the next hop address
in the Address vector. When a Target generates a DRO message, the
NH field is set to n = (Option Length - 2 - (16 - Compr))/(16 -
Compr).
o TargetAddr: This field MUST contain a unicast IPv6 address of the
Target generating the DRO.
o Address[1..n]: The Address vector MUST contain a complete route
between the Origin and the Target such that the first element in
the vector contains the IPv6 address of the router next to the
Origin and the last element contains the IPv6 address of the
router next to the Target.
9. P2P-RPL Route Discovery By Creating a Temporary DAG
This section details the P2P-RPL route discovery operation.
9.1. Joining a Temporary DAG
All the routers participating in a P2P-RPL route discovery, including
the Origin and the Target(s), MUST join the temporary DAG being
created for the purpose. When a router joins a temporary DAG
advertized by a P2P mode DIO, it MUST maintain its membership in the
temporary DAG for the Life Time duration listed in the P2P-RDO. The
only purpose of a temporary DAG's existence is to facilitate the P2P-
RPL route discovery process. The temporary DAG MUST NOT be used to
route packets. A router MUST detach from the temporary DAG once the
duration of its membership in the DAG has reached the DAG's life
time. After receiving a DRO with the Stop flag set to one, a router
SHOULD NOT send or receive any more DIOs for this temporary DAG and
SHOULD also cancel any pending DIO transmission.
9.2. Trickle Operation For P2P Mode DIOs
An RPL router uses a Trickle timer [RFC6206] to control DIO
transmissions. The Trickle control of DIO transmissions provides
quick resolution of any "inconsistency" while avoiding redundant DIO
transmissions. The Trickle algorithm also imparts protection against
loss of DIOs due to inherent lack of reliability in LLNs. When
controlling the transmissions of a P2P mode DIO, a Trickle timer
SHOULD follow the following rules:
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o The receipt of a P2P mode DIO, that allows the router to advertise
a better route (in terms of the routing metrics and the OF in use)
than before, is considered "inconsistent" and hence resets the
Trickle timer. Note that the first receipt of a P2P mode DIO
advertising a particular temporary DAG is always considered an
"inconsistent" event.
o The receipt of a P2P mode DIO from a parent in the temporary DAG
is considered neither "consistent" nor "inconsistent" if it does
not allow the router to advertise a better route than before.
Thus, the receipt of such DIOs has no impact on the Trickle
operation. Note that this document does not impose any
requirements on how a router might choose its parents in the
temporary DAG.
o The receipt of a P2P mode DIO is considered "consistent" if the
source of the DIO is not a parent in the temporary DAG and either
of the following conditions is true:
* The DIO advertises a better route than the router but does not
allow the router to advertise a better route itself; or
* The DIO advertises a route as good as the route (to be)
advertised by the router.
Note that the Trickle algorithm's DIO suppression rules are in
effect at all times. Hence, a P2P-RPL router may suppress a DIO
transmission even if it has not made any DIO transmission yet.
o The receipt of a P2P mode DIO, that advertises a worse route than
what the router advertises (or would advertise when it gets a
chance to generate its DIO), is considered neither "consistent"
nor "inconsistent", i.e., the receipt of such a DIO has no impact
on the Trickle operation.
o The Imin parameter SHOULD be set taking in account the
connectivity within the network. For highly connected networks, a
small Imin value (of the order of the typical transmission delay
for a DIO) may lead to congestion in the network as a large number
of routers reset their Trickle timers in response to the first
receipt of a DIO from the Origin. These routers would generate
their DIOs within Imin interval and cause additional routers to
reset their trickle timers and generate more DIOs. Thus, for
highly connected networks, the Imin parameter SHOULD be set to a
value at least one order of magnitude larger than the typical
transmission delay for a DIO. For sparsely connected networks,
the Imin parameter can be set to a value that is a small multiple
of the typical transmission delay for a DIO. Note that the Imin
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value has a direct impact on the time required for a P2P-RPL route
discovery to complete. In general, the time required for a P2P-
RPL route discovery would increase approximately linearly with the
value of the Imin parameter. Since the route discovery must
complete within the lifetime of the temporary DAG created for the
purpose, the Origin should set this lifetime to a large enough
value taking in account the Imin value as well as the expected
distance (in terms of hops and/or ETX) to the Target(s).
o The Imax parameter SHOULD be set to a large value (several orders
of magnitude higher than the Imin value) and is unlikely to be
critical for P2P-RPL operation. This is because the first receipt
of a P2P mode DIO for a particular temporary DAG is considered an
inconsistent event and would lead to resetting of Trickle timer
duration to the Imin value. Given the temporary nature of the
DAGs used in P2P-RPL, Trickle timer may not get a chance to
increase much.
o The recommended value of redundancy constant "k" is 1. With this
value of "k", a DIO transmission will be suppressed if the router
receives even a single "consistent" DIO during a timer interval.
This setting for the redundancy constant is designed to reduce the
number of messages generated during a route discovery process and
is suitable for the environments with low or moderate packet loss
rates. However, this setting may result in an increase in the
time required for the route discovery process to complete. A
higher value for the redundancy constant may be more suitable in
* Environments with high packet loss rates; or
* Deployments where the time required for the route discovery
process to complete needs to be as small as possible; or
* Deployments where specific destinations are reachable only
through specific intermediate routers (and hence these
intermediate routers should not suppress their DIOs).
A particular deployment should take in account the above mentioned
factors when deciding the value of the redundancy constant.
Individual P2P-RPL deployments are encouraged to share their
experience with these rules with ROLL working group to help guide the
development of standards track version of the protocol.
Applicability Statements that specify the use of P2P-RPL MUST provide
guidance for setting Trickle parameters, particularly Imin and the
redundancy constant.
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9.3. Processing a P2P Mode DIO
The rules for DIO processing and transmission, described in Section 8
of RPL [RFC6550], apply to P2P mode DIOs as well except as modified
in this document. In particular, in accordance with Section 8.2.3 of
RPL [RFC6550], a received P2P mode DIO MUST be discarded if it is
malformed according to the rules specified in this document and in
[RFC6550].
The following rules for processing a received P2P mode DIO apply to
both Intermediate Routers and the Target.
A router SHOULD discard a received P2P mode DIO with no further
processing if it does not have bidirectional reachability with the
neighbor that generated the received DIO. Note that bidirectional
reachability does not mean that the link must have the same values
for a routing metric in both directions. A router SHOULD calculate
the values of the link-level routing metrics included in the received
DIO taking in account the metric's value in both Forward and Reverse
directions. Bidirectional reachability along a discovered route
allows the Target to use this route to reach the Origin. In
particular, the DRO messages travel from the Target to the Origin
along a discovered route.
A router MUST discard a received P2P mode DIO with no further
processing:
o If the DIO advertises INFINITE_RANK as defined in Section 17 of
[RFC6550].
o If the integer part of the rank advertised in the DIO equals or
exceeds the MaxRank limit listed in the P2P Route Discovery
Option.
o If the routing metric values do not satisfy one or more of the
mandatory route constraints listed in the DIO or if the router
cannot evaluate the mandatory route constraints, e.g., if the
router does not support the metrics used in the constraints.
o If the router previously received a DRO message with the same
RPLInstanceID and DODAGID as the received DIO and with the Stop
flag set to one.
The router MUST check the Target addresses listed in the P2P-RDO and
any RPL Target Options included in the received DIO. If one of its
IPv6 addresses is listed as a Target address or if it belongs to the
multicast group specified as one of the Target addresses, the router
considers itself a Target and processes the received DIO as specified
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in Section 9.5. Otherwise, the router considers itself an
Intermediate Router and processes the received DIO as specified in
Section 9.4.
9.4. Additional Processing of a P2P Mode DIO At An Intermediate Router
An Intermediate Router MUST discard a received P2P mode DIO with no
further processing if the router cannot elide Compr (as specified in
the P2P-RDO) prefix octets from its IPv6 address or if adding its
IPv6 address to the Address vector (inside the P2P-RDO) would result
in the Address vector containing multiple, non-back-to-back addresses
belonging to this router.
On receiving a P2P mode DIO, an Intermediate Router MUST do the
following:
o The router MUST determine whether this DIO advertises a better
route than the router itself and whether the receipt of the DIO
would allow the router to advertise a better route than before.
Accordingly, the router SHOULD consider this DIO as consistent/
inconsistent from Trickle perspective as described in Section 9.2.
Note that the route comparison in a P2P-RPL route discovery is
performed using the parent selection rules of the OF in use as
specified in Section 14 of RPL [RFC6550]. If the received DIO
would allow the router to advertise a better route, the router
MUST remember the route advertised (inside the P2P-RDO) in the DIO
(after adding its own IPv6 address to the route) for inclusion in
its future DIOs. When an Intermediate Router adds itself to a
route, it MUST ensure that the IPv6 address added to the route
does not belong to an ingress-only or an egress-only interface.
To improve the diversity of the routes being discovered, an
Intermediate Router SHOULD keep track of multiple routes (as long
as all these routes are the best seen so far), one of which SHOULD
be selected in a uniform random manner for inclusion in the P2P-
RDO inside the router's next DIO. Note that the route
accumulation in a P2P mode DIO MUST take place even if the Origin
does not want any DRO messages to be generated (i.e., the R flag
inside the P2P-RDO is set to zero). This is because the Target
may still be able to use the accumulated route as a source route
to reach the Origin.
o The router MUST copy any Data Option (to be included in its future
DIO transmissions) if the received DIO comes from a parent and is
the first parent-originated DIO received with a Data Option
inside.
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9.5. Additional Processing of a P2P Mode DIO At The Target
The Target MUST determine if the received DIO contains a Data Option
and deliver the data to the specified upper layer protocol unless it
has already done so in response to a previously received DIO. If
this route discovery involves multiple Targets, the Target MUST
remember this Data Option for inclusion in its own DIOs.
The Target MAY store the route contained in the P2P-RDO in the
received DIO for use as a Source Route to reach the Origin. The
lifetime of this Source Route is specified by the Default Lifetime
and Lifetime Unit parameters inside the DODAG Configuration Option
currently in effect. This lifetime can be extended (or shortened)
appropriately following a hint from an upper-layer protocol.
If the Reply flag inside the P2P-RDO in the received DIO is zero, the
Target MUST discard the received DIO with no further processing.
Otherwise, the Target MAY select the route contained in the P2P-RDO
to send a DRO message back to the Origin. If the H flag inside the
P2P-RDO is one, the Target needs to select one route and send a DRO
message along this route back to the Origin. If the H flag is zero,
the number of routes to be selected (and the number of DRO messages
to be sent back) is given by one plus the value of the N field in the
P2P-RDO. This document does not prescribe a particular method for
the Target to select the routes. Example methods include selecting
each route that meets the specified routing constraints until the
desired number have been selected or selecting the best routes
discovered over a certain time period. If multiple routes are to be
selected, the Target SHOULD avoid selecting routes that have large
segments in common.
If the Target selects the route contained in the P2P-RDO in the
received DIO, it sends a DRO message back to the Origin (identified
by the DODAGID field in the DIO). The DRO message MUST include a
P2P-RDO that contains the selected route inside the Address vector.
Various fields inside the P2P-RDO MUST be set as specified in
Section 8.2. The Target MAY set the A flag inside the DRO message to
one if it desires the Origin to send back a DRO-ACK message on
receiving the DRO. In this case, the Target waits for
DRO_ACK_WAIT_TIME duration for the DRO-ACK message to arrive.
Failure to receive the DRO-ACK message within this time duration
causes the Target to retransmit the DRO message. The Target MAY
retransmit the DRO message in this fashion up to
MAX_DRO_RETRANSMISSIONS times. Both DRO_ACK_WAIT_TIME and
MAX_DRO_RETRANSMISSIONS are configurable parameters to be decided
based on the characteristics of individual deployments. Note that
all DRO transmissions and retransmissions MUST take place while the
Target is still a part of the temporary DAG created for the route
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discovery. A Target MUST NOT transmit a DRO if it no longer belongs
to this DAG.
The Target MAY set the Stop flag inside the DRO message to one if
o this router is the only Target specified in the corresponding DIO,
i.e., the corresponding DIO specified a unicast address of the
router as the TargetAddr inside the P2P-RDO with no additional
Targets specified via RPL Target Options; and
o the Target has already selected the desired number of routes.
The Target MAY include a Metric Container Option in the DRO message.
This Metric Container contains the end-to-end routing metric values
for the route specified in the P2P-RDO. The Target MAY include one
Data Option in the DRO message to carry time-critical application
data for the Origin, subject to the conditions listed in Section 8.
The Target MUST transmit the DRO message via a link-local multicast.
A Target MUST NOT forward a P2P mode DIO any further if no other
Targets are to be discovered, i.e., if a unicast IPv6 address (of
this Target) is specified as the TargetAddr inside the P2P-RDO and no
additional Targets are specified via RPL Target Options inside the
DIOs for this route discovery. Otherwise, the Target MUST generate
DIOs for this route discovery as an Intermediate Router would.
9.6. Processing a DRO At An Intermediate Router
If the DODAGID field in the received DRO does not list a router's own
IPv6 address, the router considers itself an Intermediate Router and
MUST process the received message in the following manner:
o The router MUST discard the received DRO with no further
processing if it does not belong to the temporary DAG identified
by the RPLInstanceID and the DODAGID fields in the DRO.
o If the Stop flag inside the received DRO is set to one, the router
SHOULD NOT send or receive any more DIOs for this temporary DAG
and SHOULD cancel any pending DIO transmission.
o The router MUST ignore any Metric Container and Data Options
contained in the DRO message.
o If Address[NH] element inside the P2P-RDO lists the router's own
unicast IPv6 address, the router is a part of the route carried in
the P2P-RDO. In this case, the router MUST do the following:
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* To prevent loops, the router MUST discard the DRO message with
no further processing if the Address vector in the P2P-RDO
includes multiple IPv6 addresses assigned to the router's
interfaces.
* If the H flag inside the P2P-RDO is one, the router MUST store
the state for the Forward Hop-by-hop route carried inside the
P2P-RDO. This state consists of:
+ The RPLInstanceID and the DODAGID fields of the DRO.
+ The route's destination, the Target (identified by
TargetAddr field inside P2P-RDO).
+ The IPv6 address of the next hop, Address[NH+1] (unless NH
value equals the number of elements in the Address vector,
in which case the Target itself is the next hop).
This Hop-by-hop routing state MUST expire at the end of the
lifetime specified by the Default Lifetime and Lifetime Unit
parameters inside the DODAG Configuration Option used in P2P
mode DIOs for this route discovery.
* If the router already maintains a Hop-by-hop state listing the
Target as the destination and carrying same RPLInstanceID and
DODAGID fields as the received DRO and the next hop information
in the state does not match the next hop indicated in the
received DRO, the router MUST discard the DRO message with no
further processing. Note that this situation would occur in
the following two cases:
+ When the route listed in the Address vector inside the P2P-
RDO contains a previously undetected loop. In this case,
the rule above causes the DRO messages to be discarded.
+ When a Hop-by-hop Route between the Origin and the Target,
previously established using the same RPLInstanceID and
DODAGID as the route currently being established, still
exists and at least partially overlaps the route currently
being established.
* The router MUST decrement the NH field inside the P2P-RDO and
send the DRO message further via link-local multicast.
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9.7. Processing a DRO At The Origin
When a router receives a DRO message that lists its IPv6 address in
the DODAGID field, the router recognizes itself as the Origin for the
corresponding P2P-RPL route discovery, notes the Target that
originated this message (from the TargetAddr field inside the P2P-
RDO) and processes the message in the following manner:
o The Origin MUST discard the received DRO with no further
processing if it no longer belongs to the temporary DAG identified
by the RPLInstanceID and the DODAGID fields in the DRO.
o If the received DRO contains a Data Option and if it has not
already done so following the receipt of an earlier DRO from this
Target, the Origin MUST deliver the data inside the Data Option to
the specified upper layer protocol.
o If the Stop flag inside the received DRO is set to one, the Origin
SHOULD NOT generate any more DIOs for this temporary DAG and
SHOULD cancel any pending DIO transmission.
o If the P2P-RDO inside the DRO has the H flag set to 0, the Address
vector inside the P2P-RDO contains a Source Route to this Target
and the Origin MUST store this Source Route in its memory. The
lifetime of this Source Route is specified by the Default Lifetime
and Lifetime Unit parameters inside the DODAG Configuration Option
in the P2P mode DIOs used for this route discovery. This lifetime
could be extended (or shortened) appropriately following a hint
from an upper-layer protocol.
o If the P2P-RDO inside the DRO has the H flag set to 1, the DRO
message is establishing a Hop-by-hop Route to this Target and the
Origin MUST store in its memory the state for this Hop-by-hop
Route in the manner described in Section 9.6. This Hop-by-hop
routing state MUST expire at the end of the lifetime specified by
the Default Lifetime and Lifetime Unit parameters inside the DODAG
Configuration Option used in P2P mode DIOs for this route
discovery. The standards track version of P2P-RPL may consider
specifying a signaling mechanism that will allow the Origin to
extend (or shorten) the lifetime of a P2P-RPL Hop-by-hop Route
following a suitable hint from an upper-layer protocol.
o If the received DRO message contains one or more Metric Container
Options, the Origin MAY store the values of the routing metrics
associated with the discovered route in its memory. This
information may be useful in formulating the constraints for any
future P2P-RPL route discovery to this Target.
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o If the A flag is set to one in the received DRO message, the
Origin MUST generate a DRO-ACK message as described in Section 10
and unicast the message to the Target. The Origin MAY use the
route just discovered to send the DRO-ACK message to the Target.
Section 11 describes how a packet may be forwarded along a Source/
Hop-by-hop Route discovered using P2P-RPL.
10. The Discovery Reply Object Acknowledgement (DRO-ACK)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID | Version |Seq| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DODAGID |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of the base Discovery Reply Object Acknowledgement
(DRO-ACK)
A DRO message may fail to reach the Origin due to a number of
reasons. Unlike the DIO messages that benefit from Trickle-
controlled retransmissions, the DRO messages are prone to loss due to
unreliable packet transmission in LLNs. Since a DRO message travels
via link-local multicast, it cannot use link-level acknowledgements
to improve the reliability of its transmission. Also, an
Intermediate Router may drop the DRO message (e.g., because of its
inability to store the state for the Hop-by-hop Route the DRO is
establishing). To protect against the potential failure of a DRO
message to reach the Origin, the Target MAY request the Origin to
send back a DRO Acknowledgement (DRO-ACK) message on receiving a DRO
message. Failure to receive such an acknowledgement within the
DRO_ACK_WAIT_TIME interval of sending the DRO message forces the
Target to resend the message.
This section defines two new RPL Control Message types: DRO
Acknowledgement (DRO-ACK; with code TBD6) and Secure DRO-ACK (with
code TBD7). A DRO-ACK message MUST travel as a unicast message from
the Origin to the Target. The format of a base DRO-ACK message is
shown in Figure 4. Various fields in a DRO-ACK message MUST have the
same values as the corresponding fields in the DRO message. The
field marked as "Reserved" MUST be set to zero on transmission and
MUST be ignored on reception. A Secure DRO-ACK message follows the
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format in Figure 7 of [RFC6550], where the base format is same as the
base DRO-ACK shown in Figure 4.
11. Packet Forwarding Along a Route Discovered Using P2P-RPL
An Origin MAY use a Source Routing Header (SRH) [RFC6554] to send a
packet along a Source Route discovered using P2P-RPL.
Travel along a Hop-by-hop Route, established using P2P-RPL, requires
specifying the RPLInstanceID and the DODAGID (of the temporary DAG
used for the route discovery) to identify the route. This is because
a P2P-RPL route discovery does not use globally unique RPLInstanceID
values and hence both the RPLInstanceID (a local value assigned by
the Origin) and the DODAGID (an IPv6 address of the Origin) are
required to uniquely identify a P2P-RPL Hop-by-hop Route to a
particular destination.
An Origin MAY include an RPL option [RFC6553] inside the IPv6 hop-by-
hop options header of a packet to send it along a Hop-by-hop Route
established using P2P-RPL. For this purpose, the Origin MUST set the
DODAGID of the temporary DAG used for the route discovery as the
source IPv6 address of the packet. Further, the Origin MUST specify
inside the RPL option the RPLInstanceID of the temporary DAG used for
the route discovery and set the O flag inside the RPL option to one.
On receiving this packet, an Intermediate Router checks the O flag
and correctly infer the source IPv6 address of the packet as the
DODAGID of the Hop-by-hop Route. The router then uses the DODAGID,
the RPLInstanceID and the destination address to identify the routing
state to be used to forward the packet further.
12. Interoperability with Core RPL
This section describes how RPL routers that implement P2P-RPL
interact with RPL routers that do not. In general, P2P-RPL operation
does not affect core RPL operation and vice versa. However, core RPL
does allow a router to join a DAG as a leaf node even if it does not
understand the Mode of Operation (MOP) used in the DAG. Thus, an RPL
router that does not implement P2P-RPL may conceivably join a
temporary DAG being created for a P2P-RPL route discovery as a leaf
node and maintain its membership even though the DAG no longer
exists. This may impose a drain on the router's memory. However,
such RPL-only leaf nodes do not interfere with P2P-RPL route
discovery since a leaf node may only generate a DIO advertising an
INFINITE_RANK and all routers implementing P2P-RPL are required to
discard such DIOs. Note that core RPL does not require a router to
join a DAG whose MOP it does not understand. Moreover, RPL routers
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in a particular deployment may have strict restrictions on the DAGs
they may join, thereby mitigating the problem.
The P2P-RPL mechanism described in this document works best when all
the RPL routers in the LLN implement P2P-RPL. In general, the
ability to discover routes as well as the quality of discovered
routes would deteriorate with the fraction of RPL routers that
implement P2P-RPL.
13. Security Considerations
A P2P-RPL deployment may be susceptible to denial of service attacks
by rogue routers that initiate fake route discoveries. A rogue
router could join a temporary DAG and advertise false information in
its DIOs in order to include itself in the discovered route(s). It
could generate bogus DRO messages carrying bad routes or maliciously
modify genuine DRO messages it receives.
In general, the security considerations for the operation of P2P-RPL
are similar to the ones for the operation of RPL (as described in
Section 19 of [RFC6550]). Section 10 of RPL specification [RFC6550]
describes a variety of security mechanisms that provide data
confidentiality, authentication, replay protection and delay
protection services. Each RPL control message has a secure version
that allows the specification of the level of security and the
algorithms used to secure the message. The mechanism defined in this
document is based on the use of DIOs to form a temporary DAG and
discover P2P routes. These DIOs can be used in their secure versions
if desired. New RPL control messages defined in this document (DRO
and DRO-ACK) have secure versions as well. In addition, a P2P-RPL
deployment may use the security features provided by the link layer
in use. Thus, a particular P2P-RPL deployment can analyze its
security requirements and use the appropriate set of RPL (or link
layer) security mechanisms that meet those requirements. Note that
the contents of the Data Option, if used, has the same level of
security as the DIO/DRO message it is part of. Hence, a P2P-RPL
deployment SHOULD take in consideration the security requirements of
the data being sent inside the Data Options when deciding the overall
security requirements.
Since a DRO message travels along a Source Route specified inside the
message, some of the security concerns that led to the deprecation of
Type 0 routing header [RFC5095] may apply. To avoid the possibility
of a DRO message traveling in a routing loop, this document requires
each Intermediate Router to confirm that the Source Route listed
inside the message does not contain any routing loop involving itself
before the router could forward the message further. As specified in
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Section 9.6, this check involves the router making sure that its IPv6
addresses do not appear multiple times inside the Source Route with
one or more other IPv6 addresses in between.
14. IANA Considerations
14.1. Additions to Mode of Operation
This document defines a new Mode of Operation, entitled "P2P Route
Discovery Mode" (see Section 6), assigned a value TBD1 from the "Mode
of Operation" space [to be removed upon publication:
http://www.iana.org/assignments/rpl/rpl.xml#mop] [RFC6550]. IANA is
requested to allocate a suitable value to TBD1. The string TBD1 in
this document should be replaced by the allocated value. The
previous two sentences should be removed before publication.
+-------+---------------------------------------+---------------+
| Value | Description | Reference |
+-------+---------------------------------------+---------------+
| TBD1 | P2P Route Discovery Mode of Operation | This document |
+-------+---------------------------------------+---------------+
Mode of Operation
14.2. Additions to RPL Control Message Options
This document defines two new RPL options:
o "P2P Route Discovery Option" (see Section 7.1), assigned a value
TBD2 from the "RPL Control Message Options" space [to be removed
upon publication: http://www.iana.org/assignments/rpl/
rpl.xml#control-message-options] [RFC6550]. IANA is requested to
allocate a suitable value to TBD2. The string TBD2 in this
document should be replaced by the allocated value. The previous
two sentences should be removed before publication.
o "Data Option" (see Section 7.2), assigned a value TBD3 from the
"RPL Control Message Options" space [to be removed upon
publication: http://www.iana.org/assignments/rpl/
rpl.xml#control-message-options] [RFC6550]. IANA is requested to
allocate a suitable value to TBD3. The string TBD3 in this
document should be replaced by the allocated value. The previous
two sentences should be removed before publication.
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+-------+---------------------+---------------+
| Value | Meaning | Reference |
+-------+---------------------+---------------+
| TBD2 | P2P Route Discovery | This document |
| TBD3 | Data | This document |
+-------+---------------------+---------------+
RPL Control Message Options
14.3. Additions to RPL Control Codes
This document defines the following new RPL messages:
o "Discovery Reply Object" (see Section 8), assigned a value TBD4
from the "RPL Control Codes" space [to be removed upon
publication:
http://www.iana.org/assignments/rpl/rpl.xml#control-codes]
[RFC6550]. IANA is requested to allocate TBD4 from the range
0x00-0x7F to indicate a message without security enabled. The
string TBD4 in this document should be replaced by the allocated
value. The previous two sentences should be removed before
publication.
o "Secure Discovery Reply Object" (see Section 8.1), assigned a
value TBD5 from the "RPL Control Codes" space [to be removed upon
publication:
http://www.iana.org/assignments/rpl/rpl.xml#control-codes]
[RFC6550]. IANA is requested to allocate TBD5 from the range
0x80-0xFF to indicate a message with security enabled. The string
TBD5 in this document should be replaced by the allocated value.
The previous two sentences should be removed before publication.
o "Discovery Reply Object Acknowledgement" (see Section 10),
assigned a value TBD6 from the "RPL Control Codes" space [to be
removed upon publication:
http://www.iana.org/assignments/rpl/rpl.xml#control-codes]
[RFC6550]. IANA is requested to allocate TBD6 from the range
0x00-0x7F to indicate a message without security enabled. The
string TBD6 in this document should be replaced by the allocated
value. The previous two sentences should be removed before
publication.
o "Secure Discovery Reply Object Acknowledgement" (see Section 10),
assigned a value TBD7 from the "RPL Control Codes" space [to be
removed upon publication:
http://www.iana.org/assignments/rpl/rpl.xml#control-codes]
[RFC6550]. IANA is requested to allocate TBD7 from the range
0x80-0xFF to indicate a message with security enabled. The string
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TBD7 in this document should be replaced by the allocated value.
The previous two sentences should be removed before publication.
+------+--------------------------------------------+---------------+
| Code | Description | Reference |
+------+--------------------------------------------+---------------+
| TBD4 | Discovery Reply Object | This document |
| TBD5 | Secure Discovery Reply Object | This document |
| TBD6 | Discovery Reply Object Acknowledgement | This document |
| TBD7 | Secure Discovery Reply Object | This document |
| | Acknowledgement | |
+------+--------------------------------------------+---------------+
RPL Control Codes
15. Acknowledgements
Authors gratefully acknowledge the contributions of the following
individuals (in alphabetical order) in the development of this
document: Dominique Barthel, Jakob Buron, Cedric Chauvenet, Thomas
Clausen, Robert Cragie, Ted Humpal, Richard Kelsey, Phil Levis,
Charles Perkins, Joseph Reddy, Michael Richardson, Zach Shelby,
Pascal Thubert, Hristo Valev and JP Vasseur.
16. References
16.1. Normative References
[PROTOCOL]
"Protocol Numbers", <http://www.iana.org/assignments/
protocol-numbers/protocol-numbers.xml>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, March 2011.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
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[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics Used for Path Calculation in
Low-Power and Lossy Networks", RFC 6551, March 2012.
16.2. Informative References
[I-D.ietf-roll-p2p-measurement]
Goyal, M., Baccelli, E., Brandt, A., and J. Martocci, "A
Mechanism to Measure the Routing Metrics along a Point-to-
point Route in a Low Power and Lossy Network",
draft-ietf-roll-p2p-measurement-08 (work in progress),
January 2013.
[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
December 2007.
[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks",
RFC 5826, April 2010.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, June 2010.
[RFC6552] Thubert, P., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)",
RFC 6552, March 2012.
[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams", RFC 6553,
March 2012.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
March 2012.
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Authors' Addresses
Mukul Goyal (editor)
University of Wisconsin Milwaukee
3200 N Cramer St
Milwaukee, WI 53201
USA
Phone: +1 414 2295001
Email: mukul@uwm.edu
Emmanuel Baccelli
INRIA
Phone: +33-169-335-511
Email: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/
Matthias Philipp
INRIA
Phone: +33-169-335-511
Email: Matthias.Philipp@inria.fr
Anders Brandt
Sigma Designs
Emdrupvej 26A, 1.
Copenhagen, Dk-2100
Denmark
Phone: +45-29609501
Email: abr@sdesigns.dk
Jerald Martocci
Johnson Controls
507 E Michigan St
Milwaukee, WI 53202
USA
Phone: +1 414-524-4010
Email: jerald.p.martocci@jci.com
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